Is this solar setup flawed?

I have a 12V 10W solar panel that I'd like to use to charge a smartphone and/or a battery pack (5000mAh-15000mAh). I realise it's not a large panel but smartphones aren't that greedy and my use would be moderate.

I was hoping that I could just attach the panel I have to a car lighter receptacle and use a double USB plug that I already have. Is this a sensible arrangement?

The advantages, to me, are that all I'd need is a $5-$10 receptacle and I'd have a very simple setup (no inverter or charge controller, well, I suppose the USB plug would be a controller, perhaps).

Would this system work? Even if it did work would it be an efficient set-up? Would there be any danger of damage to smartphone or panel?

Also: will I need to earth anything or are the devices I charge capable of earthing? Would I need a diode to stop charge flowing back to the panel when there was no sun?

Thanks for reading, I'd really appreciate any input on these questions.

The USB plug you have probably is made for voltages ranging between 6 and 14.6V, whereas a typical 12V solar panel, in full sun could be around the 20V mark (this drops with a load and with a length of wire). Test your USB plug output (some cheap ones' output will be relative to your input voltage). When loaded what is the end voltage of your solar panel? Basically, if you want it fool proof then put in a simple regulator (lots of simple designs out there), this will keep your solar panel healthy and keep the working voltage near the 14.6V mark.

Your USB plug may well be able to handle higher voltages and you can certainly "regulate" the solar panel voltage by using an appropriate length of wire (though this is not an efficient use of your energy)... really, just test what you have with your multimeter, add a simple regulator circuit if you want that peace of mind.

You ask about earthing things... voltage is all relative, so make sure your USB plug is working correctly (I would imagine that the voltage between the -ve terminal on the 5V side and the -ve terminal of the 12V side should be zero)... All the best.